1. Field of the Invention
The present invention relates to a balancing control technology for battery cells, and more particularly, to a balancing control circuit for a battery cell module using an LC series resonant circuit, which is capable of performing a balancing operation for a battery cell module using switching elements and an LC series resonant circuit.
2. Description of the Related Art
In general, when a voltage applied across a battery (battery cell) exceeds a predetermined value, the battery may explode, and when the voltage drops below a predetermined value, the battery may receive permanent damage. When power is intended to be supplied to a device requiring a relatively large amount of power, such as an electric vehicle, through a battery cell, a battery cell module (battery pack) including battery cells connected in series is used. However, when the battery cell module is used, a voltage imbalance may occur due to a difference in performance between the respective battery cells.
When the battery cell module is charged, one battery cell within the battery cell module may reach an upper-limit voltage before the other battery cells. In this case, the battery cell module cannot be charged any more. Thus, the charging must be ended in a state where the other battery cells are not sufficiently charged. As a result, the charge capacity of the battery cell module does not approach the rated charge capacity.
Furthermore, when the battery cell module is discharged, one battery cell within the battery cell may reach a lower-limit voltage before the other battery cells. In this case, since the battery cell module cannot be used any more, the duration of use of the battery cell module is reduced as much.
When the battery cell module is charged or discharged as described above, the electric energy of a battery cell having higher electric energy may be supplied to a battery cell having lower electric energy, in order to expand the duration of use of the battery cell module. Such an operation is referred to as battery balancing.
FIG. 1 is a circuit diagram of a conventional battery cell balancing circuit using parallel resistors. Referring to FIG. 1, the conventional battery cell balancing circuit includes a battery cell module 11 having battery cells CELL1 to CELL4 connected in series, resistors R11 to R14 connected in series, and switches SW11 to SW15 configured to selectively connect both end terminals of the battery cell module 11 and connection terminals between the battery cells CELL1 to CELL4 to corresponding terminals of the resistors R11 to R14, respectively.
Referring to FIG. 1, when the charge voltage of an arbitrary battery cell among the battery cells CELL1 to CELL4 within the battery cell module 11 reaches an upper-limit voltage before the charge voltages of the other battery cells during a charging operation for the battery cell module 1, a corresponding switch among the switches SW11 to SW15 is turned on to discharge the charge voltage through a corresponding resistor among the resistors R11 to R14.
For example, when the charge voltage of the second battery cell CELL2 reaches the upper-limit voltage before the charge voltages of the other battery cells CELL1, CELL3, and CELL4, the switch SW12 is turned on for a required time. Thus, while the charge voltage of the battery cell CELL12 is discharged through the resistor R12 as needed, battery cell balancing is performed.
However, when such a battery cell balancing circuit is used, power is consumed through the resistors. Thus, the efficiency is reduced as much. Furthermore, since the upper-limit voltage cannot be supplied to a battery cell having a lower voltage while the battery module is used, the efficiency is reduced.
FIG. 2 is a circuit diagram of a conventional battery cell balancing circuit using capacitors. Referring to FIG. 2, the conventional battery cell balancing circuit includes a battery cell module 21 having battery cells CELL1 to CELL4 connected in series, capacitors C21 to C23 connected in series, and switches SW21 to SW24 configured to selectively connect each of one terminal of the capacitor C21, a connection terminal between the capacitors C21 and C22, a connection terminal between the capacitors C22 and C23, and the other terminal of the capacitor C23 to one of both terminals of a corresponding battery cell among the battery cells CELL1 to CELL4.
Referring to FIG. 2, the battery cell balancing circuit using capacitors has two connection states. In the first connection state, each of one terminal of the capacitor C21, the connection terminal between the capacitors C21 and C22, the connection terminal between the capacitors C22 and C23, and the other terminal of the capacitor C23 is connected to one terminal (anode) of a corresponding battery cell among the battery cells CELL1 to CELL4, as illustrated in FIG. 2. In the second connection state, each of the one terminal of the capacitor C2, the connection terminal between the capacitors C21 and C22, the connection terminal between the capacitors C22 and C23, and the other terminal of the capacitor C23 is connected to the other terminal (cathode) of the corresponding battery cell among the battery cells CELL1 to CELL4.
However, such a battery cell balancing circuit has a problem in that the efficiency thereof decreases because a hard switching operation occurs between a capacitor and a battery cell. Desirably, the battery cells within the battery module may have the same capacity. However, due to various reasons, the battery cells have different capacities therebetween. In this case, although the charge voltage of an arbitrary battery cell is lower than the charge voltages of the other battery cells, the arbitrary battery cell may have a larger capacity. At this time, the voltage of a battery cell having a lower voltage needs to be transmitted to a battery cell having a higher voltage. However, the conventional battery cell balancing circuit cannot perform the voltage transmission function.
FIG. 3 is a circuit diagram of a conventional battery cell balancing circuit using a flyback structure. Referring to FIG. 3, the battery cell balancing circuit includes a battery cell module 31 having battery cells CELL1 to CELL4 connected in series, a flyback converter 32, switches CELL1 to CELL4 configured to selectively connect a plurality of secondary coils of the flyback converter 32 to both terminals of the battery cells CELL1 to CELL4, respectively, and a switch SW35 configured to selectively connect one side of a primary coil of the flyback converter 32 to one side of the battery cell module 31.
The battery cell balancing circuit of FIG. 3 is a battery cell balancing circuit using a flyback structure, which is one of switch mode power supplies (SMPS). The battery cell balancing circuit may transmit electric energy to the battery cells CELL1 to CELL4 connected in series within the battery cell module 31 using the switches SW31 to SW34, respectively, and may transmit electric energy between both end terminals of the battery cell module 31.
Since the battery cell balancing circuit has the shape of an SMPS, the battery cell balancing circuit has excellent efficiency. However, with the increase in number of battery cells provided in the battery cell module, the size of a magnetic core used in the flyback converter is inevitably increased. Thus, the price of the battery cell balancing circuit increases.
Furthermore, since the conventional battery cell balancing circuit does not have a function of properly controlling balancing speed, there are difficulties in improving the balancing efficiency or securing the cell stability.